1. Field of the Invention
The present invention relates to a shaft holding structure for holding a shaft, such as a rear cover supporting shaft, a strap supporting shaft of a camera or the like, in an article of resinous material.
2. Description of the Related Art
An example of a shaft holding structure of this kind is illustrated in FIG. 4, which shows a shaft for supporting a rear cover of a camera. In this example, a rear cover 1 is pivotally supported on a camera body 4 by means of a hinge 5. The rear cover 1 is held in a closed position to the camera body 4 by fastening a buckle 3 onto a metallic shaft 2 which is supported at both ends by projections 1' of the rear cover 1.
In case of the rear cover 1 which is made of resinous material, the shaft holding structure is sometimes formed by a so-called insert-molding process. That is, at the time of injection molding of the rear cover, the metallic shaft is embedded at its end portions in the resinous material of the projections 1' of the rear cover 1. FIG. 5 is a plan view showing the shaft holding structure formed by the insert-molding process according to the prior art and FIG. 6 is a sectional view taken along a line B--B in FIG. 5. In these figures, the numeral 15 indicates an impression which was formed by a positioning pin planted on an inside wall of a mold to decide the axial position of the shaft 2 at the time of injection molding of the rear cover.
The shaft holding structure formed by insert-molding according to the prior art presents a serious problem. That is, the metallic shaft tends to bend due to the contraction of the synthetic resin after molding or tends to cause cracks at stress concentrating portions of the rear cover. Explaining more in detail, it is assumed that the camera body is made of polycarbonate resin, in which glass fibers are contained in the ratio of 12%, which has thermal expansion coefficient of about 50.times.10.sup.-6, and the shaft is made of iron, which has thermal expansion coefficient of 12.times.10.sup.-6. The temperature of the mold at the time of molding is normally about 100.degree. C. Under such conditions, when the shaft having a length of 100 mm, for example, is cooled to the room temperature of 20.degree. C. after molding the rear cover of the camera, the length of the shaft and the corresponding length of the rear cover produce contractions in the F and G directions, as described below. The contraction of the rear cover made of polycarbonate is 50.times.10.sup. -6 .times.(100-20).times.100=0.4 mm, while the contraction of the shaft made of iron is 12.times.10.sup.-6 .times.(100-20).times.100=0.096 mm, with the result that a dimensional difference of about 0.3 mm is produced therebetween. However, in the example as shown in FIGS. 5 and 6, in which the shaft 2 is wholly enclosed at its both ends by the resinous material and completely connected to the latter, the above-mentioned dimensional difference produces an internal stress, which tends to produce bending of the shaft 2 or cracks 17 at the corners 16 of the bifurcated projections 1' of the rear cover 1, as shown in FIG. 7.
The above-mentioned problem is not limited to the case of the camera but is generally caused in the case of an article made of resinous material having a shaft holding structure in which a metallic shaft is held at its both ends in the resinous material by insert-molding.